This invention relates to a magnetic marker or tag and to methods of position and orientation detection using the marker or tag. It will be appreciated that the terms xe2x80x9cmarkerxe2x80x9d and xe2x80x9ctagxe2x80x9d are used herein interchangeably; the term xe2x80x9clabelxe2x80x9d is also used in the art to describe magnetic articles of the type to which this invention relates. It should also be noted that the term xe2x80x9cmagneticxe2x80x9d is used herein in the sense that the tag undergoes some kind of detectable change when subjected to appropriate magnetic conditions; the term does not imply that the tag is ferromagneticxe2x80x94in general, the tag will not display ferromagnetic properties. Typically, the magnetic materials used for such tags are soft magnetic materials; these may display anisotropic magnetic properties, usually possessing a preferred direction of magnetisationxe2x80x94i.e. an axis along which the material may readily be magnetised; the magnetic permeability along this preferred axis is much greater than in other directions.
A tag in accordance with this invention can be added to existing equipment, for instance it may be secured to the tip of a catheter, which is used in conjunction with special interrogation equipment. The essence of the invention is the provision of a uni-directional tag, which avoids the 180 degree ambiguity usually found with the existing state-of-the-art magnetic tags.
EAS (electronic article security) systems use a magnetic material as a marker or tag, which is attached typically to retail articles. Typically the tag is detected by a pair of coils when the tag passes between them. These EAS systems utilise the magnetic (induction) characteristics of the material used in the tag for the purpose of detection.
A more advanced system is able to detect not only the presence but also the location and orientation of the tag or market. An example of an application where this is beneficial would be catheter location, where the magnetic material (tag) can be directly sputtered onto existing catheters. The position and orientation of the catheter can then be determined using external systems.
Existing tags have suffered from being bi-directional; that is their orientation is ambiguous since the detection signal is the same after rotating the tag through 180 degrees as it was initially. Thus the presence of the tag may readily be detected, but its orientation is undefined since it could be positioned in either of two possible pointing directions.
Alternatively the tags have incorporated hard magnetic material, which can be affected by large external field levels and the tag performance is then deranged.
The invention provides a tags which has been designed so that in the presence of a magnetic gradient field, it exhibits a non-symmetric MH loop. Hence the pointing direction of the tag can be detected. (Notexe2x80x94it is customary to describe magnetic material properties as xe2x80x9cBHxe2x80x9d loop, whereas this patent will describe the MH loop, where M is magnetisation).
According to one aspect of the present invention there is provided a tag which is characterised in that the saturation magnetisation of the tag material is not a constant value at all points along the tag.
This invention describes a tag whose orientation is unambiguous and includes features that the interrogation equipment can process to yield pointing direction. The tag can be constructed so that it can survive exposure to high magnetic fields typically found in MRI (magnetic resonance imaging) machines without affecting its unidirectional behaviour. Thus the tag is generally formed of a material which does not undergo permanent change when exposed to high magnetic fields.
Generally, the tag has a main axis and in that the saturation magnetisation at one end of said axis differs from the saturation magnetisation at the opposite end of said axis. Conveniently, the saturation magnetisation is a function of position along said axis. One way of achieving this is for the tag to be tapered in shape. For example, the tag may be triangular in cross-section. In one embodiment, the tag is generally elongate and is wider at one end than at the other. In another embodiment, the tag is generally elongate and has a thickness which is greater at one end than at the other. A tag of this form may be constructed by laminating material to achieve variation in thickness. In a third embodiment, the tag is generally elongate and is tapered in both the width and thickness directions.
A tag in accordance with this invention may be formed from a spin melt ribbon or from thin film material. Alternatively, the tag is formed from thin mu-metal sheet.
It is also possible to make the tag by sputtering a material directly onto a carrier whose position is to be detected.